Fail to get permalink when generating `og:url` with it

[zacky1972]

I wrote the following _layout.slime to generate OGP meta data:

doctype html
html
  head
    title TITLE
    meta charset="UTF-8"
    meta name="viewport" content="width=device-width, initial-scale=1.0"
    = link_to_css @env, "css/theme.css", media: "all"
    meta name="twitter:card" content="summary"
    meta property="og:url" content=@permalink
    meta property="og:title" content=@title
    meta property="og:description" content=@description
    meta property="og:image" content=@image
  body
    = @children

I guess @permalink will generate the permalink of the page, but I got the following warning:

warning: assign @permalink not available in EEx template. Please ensure all assigns are given as options. Available assigns: [:env, :description, :extension, :title, :image, :children, :input_file, :output_file, :dependency_chain, :preprocessor]
  (eex 1.14.4) lib/eex/engine.ex:140: EEx.Engine.fetch_assign!/2
  deps/still/lib/still/preprocessor/slime/renderer.ex:13: R41744.Still.Preprocessor.Slime.Layout.render/0
  (still 0.8.0) lib/still/preprocessor/slime.ex:22: Still.Preprocessor.Slime.render/1
  (still 0.8.0) lib/still/preprocessor/slime.ex:12: Still.Preprocessor.Slime.run/2
  (elixir 1.14.4) lib/enum.ex:4249: Enum.flat_map_list/2
  (still 0.8.0) lib/still/preprocessor/pagination.ex:29: Still.Preprocessor.Pagination.run/2

How should I do?